Lecture Notes on SIZING



Lecture Notes on SIZING

"No amount of genius can overcome a preoccupation with detail"

Murphy's law

To estimate the time it takes to do a task, estimate the time you think it should take, multiply by two, and change the unit of measure to the next higher unit. Thus allocate 2 days to do a one-hour task.

The law of optimum sloppiness

For any problem there is an optimum amount of sloppiness we can use to solve the problem.

KISS: Keep it simple, stupid

Corollary:

"There are occasions when we must be sloppy or imprecise in our calculations, and there are times when we must be precise. The essence of engineering is to be only as complicated as you have to be, but you must also be able to get as complicated as the problem demands".

Separation Tower Design

Distillation:

Absorption:

Extraction:

Adsorption:

Sizing Problem

# of stages

Type of column

Height, Diameter, Cost

Shell Thickness & weight

Utility requirements, Operating Cost

Types of Equipment

Plate Columns Packed Columns

(Finite stage contactors) (continuous contactors)

[pic]

Sieve Trays Packing type

Bubblecap Liquid redistributer

Valve Trays

Downcomer

Packed versus Plate Tower

Packed Tower

• Diameter < 4 ft

• Cannot handle dispersed solids in feed

• No interstage cooling

• Limited operating range

• not suitable for large temperature variations

• cheaper to construct

• design database is poor

• cheaper if corrosive fluids are involved

• pressure drop is smaller (good in vacuum operation)

McCabe – Thiele Diagrams

[pic]

[pic]

[pic]

[pic]

Preliminary Design of Columns

1. Column Pressure and Temperature

Reboiler temp [pic] boiling point of heavy component

Condenser temp [pic] boiling point of light component

Increasing column pressure: increases both temp.

decreases relative volatility

and hence make separation

more difficult

Considerations: Are utilities available at condenser and reboiler?

2. Selection of key components

3. No. of stages (Fenske-Underwood-Gilliland Method)

Used in DSTWU

1. Assume 99% LK goes overheard, 99% HK goes in bottoms. All components lighter goes with LK. Heavier goes with HK.

2. Do a material balance on column. Determine mole fraction of light key in Distillate, (xLK)D etc

3. Penske equation

[pic]

4. Underwood Equation

[pic] : solve for [pic]

[pic] Compute minimum reflux ratio

5. Gilliland Correlation

Solve For

4. Plate Efficiency and Column Height

See Perry for one correlation

Assume 50% if no info is available

[pic]Actual # of trays = [pic]

Tray Spacing = 24”

Smaller for tall columns

Height = 24” x # of trays

5. Column Diameter

[pic]

[pic]

vapor flow = L + D L = Reflux

D = Distillate

Typical Velocities (of Vapor Flow)

Atmospheric 3 ft/sec

Vacuum 6 – 8 ft/sec

Pressure 1 ft/sec

Care must be taken in vacuum operation to minimize [pic] across trays.

6. Utility Requirements

[pic]

Auxiliary Equipment Needed for column

Absorbers & Strippers

[pic]

Kremser Equation

Packed Tower Design

Empirical Correlations available for HETP

Height = # of stages [pic] HETP

Diameter fixed by vapor velocity

See Perry for correlation

• flooding

• channeling

Heat exchanger sizing

Problem:

Given: Flow rate and inlet and outlet temperature of the stream to be heated or cooled

Compute: Type and area of heat exchanger, Utility requirements, Pressure drop.

References:

Peters and Timmerhaus, pp. 528-573

D.Q. Kern, Process Heat Transfer

Perry's Handbook

Types of Heat Exchangers

Double pipe heat exchanger

Shell and Tube

Extended surface

Coiled tube

Air-cooled

Selection of Tubeside fluid

Corrosive fluids

Fluid with greater fouling tendency

Fluid at higher pressure

Less viscous fluid

Heat Exchanger Geometry

Lengths: 8, 12 and 16 ft standard

Tube dia: 3/4 or 1 inch

Tube wall thickness: Depends on pressure

Baffle spacing: ~ shell diameter

Utility Selection

Cooling Medium Cooling water 75-110 F Return at 115 to 125 F

Chilled water 40 F

Refrigerant < 32 ( Freon, Propylene)

Dowtherm for higher temperature

Waste heat boiler ( at higher temperatures)

Heating Medium

Low pressure steam 0-15 psig 250-275 F

Medium pressure steam 15-150 psig, 360 F

High pressure steam < 500 psig, 450 F

Dowtherm < 750 F

Fused Salt < 1100 F

Direct Fire > 450 F

Short cut methods for HX design

Assume countercurrent flow

3/4 in OD tubes, 8 ft length

< 10,000 sq.ft. area per exchanger

Assume 15-20 F min approach temp.

If necessary optimize area by adjusting outlet temp of utility

Use tables and graphs for U.

Keep Q/A < 12,000 Btu/hr/sq.ft in reboilers

For coolers use max water outlet temp permissible

For air-coolers use 20 hp per 1000 sq.ft of area. Air inlet at 90 F. Temperatue approach 40 at outlet

Pipe Design

Factors:

• Diameter of pipe

• Wall thickness

Pipe diameter

Small dia [pic]

Large dia [pic] higher cost

See Perry for correlations

Use friction factor charts to estimate [pic]

[pic]

Pipe Wall Thickness

[pic]

Typical Schedule # 40, 80

Nominal vs. Actual

Pumps: Pressure change in liquids

Theoretical Horse Power: (THP)

Computed from Mechanical Energy Balance

Brake Horse Power = [pic]

1. Centrifugal Pumps

• 15-5000 gpm

• 500 ft max head[pic]

2. Axial Pumps

• 20-100,000 gpm

• [pic]40 ft head

eg. Bike pumps

3. Rotary Pumps

• 1,500 gpm

• 50,000 ft head

[pic]

4. Reciprocating Pump

• 10-10,000 gpm

• 1,000,000 ft head

NPSH : Net Positive Suction Head 1-2 m of liquid

[Pin – Pvp]

Pressure change in gases

• Fans

• [pic][pic]

• Blowers

• Compressors

• [pic] [pic]

• Ejecters for vacuum

[pic]

Single stage versus Multistage

Interstage cooling needed

Pressure Vessels

Includes: Flash Drums, Reactors, Tanks, Column shell etc.

[pic]

Structural Rigidity [pic] Min wall Thickness

Flash Drums: [pic]

5 min holdup time (liquid)

Diameter based on gas velocity

[pic]in flash drums

used for Reactors

Flash Drums

Feflux Drum

At low pressures and large volumes, use storage tanks

Chemical Reactors

Factors Affecting Choice

1) no. of phases present

2) Pressure

3) Temperature

4) Residence time

5) Conversion

6) heat effects

Specify

i) Volume of reactor

ii) geometry

iii) heat transfer

iv) agitation

v) material of construction

1. Homogenous Gas Phase

- multiple empty tubes in parallel

- fast reaction , 1 sec ras. Time

- strong heat effects

furnaces for endothermic

diluent for exothermic

small die for exothermic

2. Homogenous Liquid Phase

- CSTR for low to med conversions, slow reactions

(better heat transfer)

- Plug flow for faster reactions, high conversion

- combination may also be used

3. Hetero – liquid/gas

- stirred vessels with baffles/agitation

- use gas velocity

0.2 ft/sec if gas is mostly absorbed

0.1 ft/sec if gas is 50% absorbed

0.05 ft/sec if gas is mostly not absorbed

4. Liquid/Solid

- well-stirred CSTR

- slurry reactors

5. Solid/gas

- packed types (solid not consumed)

- fluidized bed

- spouted bed

Materials of Construction

Carbon steel, most commonly used

• Not suitable for dilute acids or alkaline solutions

• Brine, salts will cause corrosion

• Not suitable at high or cryogenic temperatures

Stainless Steel

• Type 302, 304, 316 common

• Corrosion resistance

• High temperature strength

Copper good for alkalies

Nickel clad steel: Caustic materials

Glass lined steel:

Plastics: Moderate temperatures ................
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